Many manufacturing and construction processes result in the production of waste water. Environmental considerations require that waste water containing high levels of solids produced as a by-product from manufacturing or construction processes be treated to reduce the level of solids and other contaminants to an acceptable level prior to the disposal or reuse of the waste water. Statutory authorities regulate the level of contaminants that may be present in waste water disposed of through storm water, grey water or sewerage systems. Prior to such disposal of waste water it is often necessary to remove one or more contaminants whereby the waste water is of a quality suitable for disposal. In any process where contaminants are removed from waste water the contaminants need also to be disposed of. The disposal of such contaminants may be costly or technically difficult.
In many manufacturing processes waste water is produced as a by-product and has relatively high levels of suspended solids. The suspended solids may be removed by filtration or other processes to an acceptable level. However, the filter cake or waste that is produced by these processes requires disposal. Waste water containing other contaminants may require treatment by other processes to remove contaminants. However, the contaminants removed from the waste water require separate disposal. The disposal of contaminants removed from waste water is often regulated and there disposal imposes a cost or technical burden on the operator.
In the manufacture of ready mix concrete large volumes of water are consumed in production as well as in the cleaning of the production facility and wet concrete transport vehicles. The cleaning of the facility and vehicles is critical to the production process so that wet concrete is removed before it sets and becomes far more difficult to remove.
Concrete transport vehicles typically return about 3 to 5% of the wet concrete to the production facility, the majority of which is dumped in landfill and the remainder rinsed from vehicles which produces contaminated water as well as a small amount of aggregate that cannot be economically reused. It is not feasible to recover all, or more, of the aggregate as this will generate far greater volumes of contaminated water that needs to be filtered prior to disposal.
It has not generally been possible to economically recycle this contaminated water back into the manufacturing process as it is heavily contaminated with fines that when incorporated into a concrete blend increases the quantity of cement required to obtain the desired strength in the concrete. Furthermore, the presence of high quantities of fines leads to slumping of the concrete as the fines retain excessive water and also results in cracking of the concrete. Accordingly, in the manufacture of ready mix concrete only a very small quantity of water has been, or in fact can be, returned to the manufacture of concrete.
Other construction processes face similar challenges when dealing with waste water contaminated with excessive quantities of solids. For example in the production of foundations the water used to carry illminite is generally town water as the waste water produced is often too high in contaminants. In tunnelling processes water is often used as a cutting fluid and contaminated water is not generally recycled unless it has been passed through a filter press or other process for the removal of substantially all of the contaminants. It will be appreciated that by removing substantially all of the contaminants a large quantity of waste is produced.
We have now found a process for operating a water recovery plant associated with a manufacturing or construction process whereby the quantity of waste produced for disposal may be reduced.